I hate to say this to you but all of your loads were supersonic. At 10 grains of 700-X you will probably be around 1,400 fps plus or so which is way over supersonic. Your 25 grains of R-15 was probably around 1,800 fps plus.Since supersonic is right around 1,100 fps you would honestly have to load well below 10 grains of anything to be below that. Just like with trying to use visual signs for over pressure, which can be very unreliable, the sound of the supersonic crack is not a reliable cue if you don't detect it.

Agreed. Hard to tell without a chrono. All I know is that my 42.5 load makes a loud crack and my 18 load makes a pop like a cork gun with practically no recoil. The bullet drop is about 12 inches compared to the jacketed load. I still have about 50 cast bullets left. I'm waiting for the .312 bullets I ordered from Beartooth on 12/18 but haven't heard from them since.

...my 18 load makes a pop like a cork gun with practically no recoil. The bullet drop is about 12 inches compared to the jacketed load...

Welcome to the world of Milsurps shooting reduced lead loads. Since most Milsurps shoot high as they are usually sighted in for somewhere around 200 yards or so at a minimum, having a 12" drop is a good thing. Most of my Milsurps all hit to POA or with some I have to aim to 6 o'clock which is more than acceptable. As long as you are getting the bullet to stabilize and give you decent accuracy it should hit close to POA at 100 yards.

I am just now experimenting with using Trail Boss in these Milsurps. So I am hoping to be shooting ust around supersonic velocity. My goal is to try and get subsonic and still stabilize the bullet. Then we'll see what kind of accuracy I am getting. My wife just gave me a Magneto Speed so now I can verify what my loads are actually doing and what it takes to go subsonic. I am going to be doing some special loads today or tomorrow.

So many variables it's hard to know your combo. Nice 303 rifle. As mentioned earlier you don't need to full length resize once you fire the new rounds as long as you only use them in the same rifle. 303 Brit is known to have a sloppy chamber for reliability of using ammo from various war manufacturers. Also, the accuracy of any rifle actually depends on the last 2 or 3 inches of the bore being good. It could be your old 303 has a muzzle bore ruined by years of cleaning using the std military steel sectioned cleaning rods rubbing against the bore. Many old M1 Garands had this issue.
Most 303 loads I played with only required a standard large rifle primer (not Magnum rifle primer). On a 303 Brit the case is a rimmed one and the headspace is on the rim just like a Win 30-30-----so the headspace should be more forgiving of your sizing.

Try only neck sizing and seat the bullets just to the bottom of the case neck. Also, it was not clear on what size bullet you were using----for a 303 it should be a .312 diameter bullet (not a 308 diam). If you were using 308 that would account for the large grouping on the targets. You don't need a hot load for the 303----a good medium load can be accurate and comfortable to shoot.

Update to the .3115" cast bullets. First, I used a cracked empty case to measure the seating depth in the MN9130 and got 2.70" which is massively short but easier to lock the bolt down now.

I tried 700-x powder from 10 gn to 14 gn based on the Lyman cast bullet manual. I mostly hit paper but wasn't accurate. I got one in the red. Probably a lucky shot since the next didn't hit paper.

There were no pressure signs so went to 16 and 18 gn. Still not accurate but with less bullet drop. There was primer cratering with no other pressure signs. The report was cracking instead of popping like the lighter loads.

Don't think there's any point in continuing with the 700x. Does anyone want to trade for a bottle of Reloader 7?

Update to the .3115" cast bullets. First, I used a cracked empty case to measure the seating depth in the MN9130 and got 2.70" which is massively short but easier to lock the bolt down now.

I tried 700-x powder from 10 gn to 14 gn based on the Lyman cast bullet manual. I mostly hit paper but wasn't accurate. I got one in the red. Probably a lucky shot since the next didn't hit paper.

There were no pressure signs so went to 16 and 18 gn. Still not accurate but with less bullet drop. There was primer cratering with no other pressure signs. The report was cracking instead of popping like the lighter loads.

Don't think there's any point in continuing with the 700x. Does anyone want to trade for a bottle of Reloader 7?

Have you slugged your barrel yet? Without that little tidbit of knowledge your accuracy problems could be the incorrect diameter boolit. You will want to remove any and all other variables before blaming the powder.

Are your boolits keyholing at all? Just curious if you are seeing this.

Not yet. A .312 does not pass through the barrel. Anything bigger than .312 is worthless to me until I catch the casting bug (which it hasn't yet). I have 25 of the .3115" cast bullets left. I'd like to try R7 but would rather trade for the 700x.

Not yet. A .312 does not pass through the barrel. Anything bigger than .312 is worthless to me until I catch the casting bug (which it hasn't yet). I have 25 of the .3115" cast bullets left. I'd like to try R7 but would rather trade for the 700x.

What weight bullet are you using?

Not yet. A .312 does not pass through the barrel. Anything bigger than .312 is worthless to me until I catch the casting bug (which it hasn't yet). I have 25 of the .3115" cast bullets left. I'd like to try R7 but would rather trade for the 700x.

I am not sure how not being able to drop a .312" diameter bullet into your barrel tells you anything? The .312" diameter bullet you are trying to drop into your barrel is being held back by the lands of your barrel. The lands should be around .308" in diameter. So of course your bullet won't drop in. You want to slug your barrel to determine the diameter of the grooves. The grooves for your mosin are probably somewhere around .312" to .315". And you use this diameter to determine what size cast boolit to use.

The rule of thumb from the cast boolit website is groove diameter plus 0.001" for cast boolits. So if your groove diameter is .314" you need to use a .315" boolit. The rule of thumb for jacketed bullets is groove diameter. So if your groove diameter is .314" you should be using a bullet that is .314". These recommendations are for optimum performance. If you go below the recommendations you run the risk of the bullet not stabilizing and therefore not being accurate. And if you go above the recommendations you run the risk of an over pressure situation. I am not saying these recommendations are set in stone. YMMV